Symposium on Land Subsidence; Land subsidence: proceedings ...hydrologie.org/redbooks/a088/088017.pdf · Land subsidence in Osaka Water Works of Osaka is also illustrated. The peak

Land subsidence in Osaka

a certain parameter which controls the ground-water flow. One of the parameters consid- ered as the controlling factor is the water permeability of the aquifer, multiplied by the gradient of ground-water pressure, which describes the lateral flow of ground water according of Darcy’s law. The distribution of Dij obtained above may therefore be some indication of the distribution of the ground-water flow or that of the water permeability within the aquifer concerned. If so, the distribution of Dij may have some relation to the underground geological features, since the water permeability within the aquifer is depend- ent on the size of particles which constitute the soil layer and its compactness.

Referring to the geological map of Tokyo [2], the geological structure in the region under consideration is such that the water bearing sandy layer of diluvial deposits is covered with the clayey soil layer of alluvial deposits.

Therefore, the configuration of contour lines which represent the surface topography of diluvium, shown reproduced in figure 6, may have some relation to the distribution of

The fact that there are several regions where the values of Dij are in turn positive and negative, cannot yet be explained. When this question is answered, the solution of the problem of unusual distribution of land subsidence will be approached. The complete explanation of areal extension of land subsidence will be approached by further advance- ment of research on the related phenomena, such as the behaviour of the ground water on the basis of water balance and so on.

In conclusion, the authors wish to express their sincere thanks to the colleagues and the staff members of Tokyo Institute of Civil Engineering for their kind assistances in preparing this report. Dij given in figures 4 and 5. On comparing the distribution of Dij with the diluvium surface topography, it may be noted that the areas of negative Di tend to be concentrated in the zone of the diluvial valley, and in the coincident industrial centre, where the with- drawal of the ground water is most actively carried on. It is also observed that, in several parts of the region, the values of Dij are always positive, while, in some other parts, the values of Dij are sometimes positive and sometimes negative, though the diluvium topo- graphic features which favour the distribution of Dij is not observable.

REFERENCES 1. WADATI, Kiyoo (1940): “Land Subsidence in West Osaka, II”, (Japanese), Report of Research

Institute for Disaster Preuention, No. 3. 2. Tokyo Institute of Civil Engineering (1969): “Geological Map of Tokyo”.

LAND SUBSIDENCE IN OSAKA

S. MURAYAMA Dr. Eng., Professor, Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan

ABSTRACT One of the typical examples of land subsidence in Japan can be seen in Osaka city,

where the maximum subsidence amounts to about 2.5 m during the last 25 years. Such remarkable ground subsidence is due mainly to the consolidation of clay strata caused by the decrease in the artesian pressure. This can be confirmed by the results of the following investigation. * Dr. Eng., Professor, Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan.

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S. Murayama

(1) There was a close correspondence between the level of ground water and the actual rate of subsidence. When the water level recovered, the subsidence became slight and sometimes it stopped.

(2) The distribution of the pre-consolidation pressure in the normally consolidated alluvial clay in Osaka shows the decrcase in the artesian head in the aquifer. In the Symposium the following topics on the land subsidence in Osaka will be

reported: (a) History and outline of the land subsidence; (b) Geological constitution and some physical properties of earth materials; (c) Review of research on the land subsidence in Osaka; (d) Field instrumentation and the results measured; (e) Regulation against the use of ground water and its result on the land subsidence.

R~SUMÉ L‘un des exemples typiques d’affaissements au Japon se trouve à Osaka, où la

subsidence maximale s’est élevée à environ 2,5 m au cours des 25 dernières années. Cet affaissement est dû en ordre principal à ia consolidation des couches d’argile causée par la diminution de la pression artésienne. Ceci peut être confirmé par les résultats des recherches suivantes : 1. I1 y a une correspondance étroite entre le niveau dc l’eau souterraine et le taux actuel

d’affaissement. Quand le niveau de l’eau remonte, l’affaissement devient faible et parfois s’arrête.

2. La distribution de la pression de pré-consolidation dans l’argile alluviale normalement consolidée montre la diminution de la pression artésienne dans l’aquifère. Les points suivants sur les affaissements à Osaka seront présentés :

a) Histoire et description des affaissemcnts ; b) Constitution géologique et quelques propriétés physiques des échantillons de sol ; c) Revue des recherches sur les affaissements à Osaka ; d) Instrumentation et résultats de mesures ; e) Mesures régulatrices de l’usage de l’eau souterraine et leurs résultats sur les affais-

sements.

1. HISTORY AND OUTLINE OF THE LAND SUBSIDENCE

Osaka, the second largest city in Japan, has developed on an alluvial plain of ancient sediment from the Rivers Yodo and Yamato. The alluvial layer consists in the upper part of a sand stratum several meters thick, originally deposited as the top bed of a recent delta, and in the lower part as an alluvial clay. The average thickness of the clay layer is about 15 m, but it becomes thicker as it goes nearer to the coastel zone. Below the Alluvium there is a very thick Diluvium deposit of Pleistocene age, which consists of alternating layers of sand and clay.

In early years, the ground water level in the City was very high and it is reported that even artesian wells could be found in some parts of the City. However, the use of ground water was gradually increased and land subsidence due to the withdrawal of the ground water began to appear.

In the period from 1885 to 1928, when pumping of underground water was not so heavy as in later years, the rate of subsidence in the City was very slight, holding at the almost uniform rate of 6 to 13 mm per year. This slight subsidence is considered to be the result of the natural movement of the earth’s crust and the natural consolidation of the newly deposited alluvial clay. After 1928, however, the rate of subsidence increased markedly with increasing use of underground water. Investigating the oíd data of leveling, the presage of the subsidence due to withdrawal of underground water can be seen in about 1928. This date almost coincides with the time when the use of underground water for industrial purpose became active. Subsequently, a remarkable rate of subsidence began to appear in about 1934, From this period, the precise leveling for the wider ,part of the City was frequently performed by the Municipal Authorities, following the suggestion of

106

Land subsidence in Osaka

Year

Annotat ion : OP: meanr tha lowst law wotrr IwaI observed In Osaka Port in 1- and this watrr level Is wad 01 the Standard Irvcl in tho Omka Arra.

FIGURE 1-1. Annual Variation of ground water level and amount of land subsidence in Western Osaka

Prof. M. Imamura. Besides such leveling, the consolidated amount of the soil layer and the artesian head in the aquifer at 0.P.l-176 m were observed by the apparatus which had been installed by Dr. K. Wadachi at Kujho-Park in the City in 1938. Figure 1-1 shows the amount of land subsidence of various bench marks in Western Osaka and the annual variation of the artesian head at the Kujoh-well stated above. From this figure, the annual amount of land subsidence in the early period amounts to more than 15 cm. Figure 1-2 shows the monthly variation of the quantity of the water pumped in Osaka City, the variation in artesian head, and the rate of land subsidence measured at the Kujho-well. In figure 1-2.1, the quantity of industrial water supplied by the newly established Industrial

1. O.P. (Osaka Peil) means the lowest low-water level observed in Osaka Port in 1885 and this level is used as the standard datum in Osaka area.

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S. Murayama

a Year

FIGURE 1-2.1. Monthly variation of water quantity pumped from wells and variation of water buantity supplied by industrial waterworks in Osaka City

Year

FIGURE 1-2.2. Monthly variation of ground water leuel obseroed at Kujoh Park in Osaka

E - E Year

FIGURE 1-3.3. Monthly variation of land subsidence observed at Kujoh Park in Osaka

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Land subsidence in Osaka

Water Works of Osaka is also illustrated. The peak points in figure 1-2.1 and figure 1-2.2 lie in 1961, while that in figurz 1-2.3 lies in 1958. In spite of some unconformity in the details, however, the general variation in the artesian water level and that in the rate of subsidence show a close correspondence and suggests the existence of strong relationship between both phenomena. In figura 1-2.3, the period when land subsidence stopped corresponds to the period of recovered level of the underground water (fig. 1-2.2). This was caused by pumping bsing stoppsd due to the destruction of the City by the heavy bombing in the World War II. The total amount of subsidence during thirty five years from 1935 to 1968 in Osaka is shown by the isopleth in figure 1-3. The figure shows that the subsidence is 1arg:r nearzr the coastul zone, and that a zone of almostno subsidence is left in the hilly part in the middle of Osaka, where the very thin alluvial layer is covered.

The pumping wells hava various depths, but most wells are shallower than -200 m. The use of underground water fluctuates every day, every week, and every season. This

(unit :CF)

FIGURE 1-3. Isopleth of amount of land subsidence in Osaka City from 1935 to 1968

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S. Murayama

is reflected in the artesian water level-levels during the night, Sunday and winter were mostly higher than usual.

Due to the remarkable land subsidence stated above, the ground height of a part of Western Osaka has lowered below sea level and the City is exposed to the danger of flooding caused by a high tide in Osaka Bay. Figure 1-4 shows the ground height of Western Osaka in 1961.

The flood feared was unfortunately realized in 1934 by the Muroto Typhoon, one of the biggest typhoons which have ever attacked Osaka. As a result an area of about 49 sq k m was flooded by the high tide of O. P. + 4.20 m.

FIGURE 1-4. Ground height of Osaka City in 1961

'1 10

Land subsidence in Osaka

In order to prevent such a disaster in the future, more satisfactory dikes have been repaired and built. Besides these prevention works, the use of underground water has gradually been regulated in phase with the completion of the industrial water-supply works planned as the substitute of the underground water. (see fig. 1-2.1).

Owing to the regulation against the use of ground water in Osaka City land subsidence in the City gradually decreased and is almost stopped at present. This result can be recognized by comparing figure 1-3 and figure 1-5; the latter shows the isopleth of the amount of land subsidence from 1963 to 1968.

In spite of the success in preventing the land subsidence in Osaka City, however, the land subsidence in Eastern and Northern Osaka has increased remarkably during'the last few years because these regions have been developed lately and many factories using much

FIGURE 1-5. Isopleth of amount of land subsidence in Osaka area from I963 to 1968 (unit: cm)

underground water have been built. Figure 1-6 shows the amount of land subsidence in Eastern and Northern Osaka measured since 1965 at several bench marks. This figure shows that the subsidence at Nagase observation station come sup a remarkable 220 IMI during only three years. Figure 1-7 gives the monthly variation of ground water level and

1 1 1

S. Murayama

the rate of land subsidence of the several observation stations in Eastern and Northern Osaka. From figures 1-7.1 and 1-7.2, it seems that the shallower the ground water level is, the less the rate of land subsidence becomes.

In order to prevent land subsidence in Eastern and Northern Osaka, the use of under- ground water for industrial purposes has been regulated by law since 1965 in the Northern

!34 5f

Yeoi

@@- :i I

FIGURE 1-6. Amount of land subsidence in Northern and Eastern Osaka

region and since 1966 in the Eastern region. The industrial water supply service to substi- tute for the underground water has been proceeding and the preventive effect should be realized in the near future.

The annual land subsidence from 1967 to 1968 is illustrated in figure 1-8. This figure shows that the land subsidence in Osaka City has almost stopped except for the

?! oq u if BI

FlGURE 1-7.1. Monthly variation of ground waier level in Northern and Eastern Osaka

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Land subsidence in Osaka

coaste1 zone arround the Port of Osaka. However, the land subsidence in Eastern Osaka is still active and there is a large area of land subsidence whose maximum amount is 16 cm. Besides these land subsidences, it can be found that a new area around Kishiwada City in the Southern Osaka is developing. Though the size of the area is still very small, it may be important to pay close attention to it.

Year

FIGURE 1-7.2. Monthly variation of land subsidence in Northern and Eastern Osaka

In order to protect the City from high tides, as described previously, high-tide preventive dikes with a height 5 m above O.P. were constructed along the bay coast and both sides of rivers running through the City by the Osaka Prefectural Government and the Osaka Municipal Office. The total length of those dikes amounts to about 124 kilometers.

Furthermore, in order to withstand a severe high tide caused by an extra large typhoon, the Osaka Prefectural Government now has a project to raise the height of the existing high-tide preventive dikes up to 6.6 m above O.P. and is constructing high-tide preventive locks across the rivers at the most effective down-stream sites.

2. G E O L O G I C A L CONSTITUTION AND SOME PH Y S I C A L PROPERTIES OF EARTH M A T E R I A L S

(1) SUBSURFACE GEOLOGY OF OSAKA BASIN

By correlating the deep boring logs and geological investigations, the generalized strati- graphy of the Osaka Basin can be shown in table 2-1. As for the shallow part of the subsurface structure, its geological profile along the line A-A in figure 1-3 is shown in figure 2-1. As shown in this figure, the alluvial layer of about 20-30 m thick is covering the main part of the ground surface. Beneath the alluvial layer is a diluvia1 layer which belongs to the Pleistocene age and is of thicknesses up to several hundred meters, as shown in the boring logs of figure 2-2. In the pli0 and middle- early Pleistocene sediments, there generally exist 10 marine clay layers which are numbered in series as Ma-1, Ma-2, . . . and Ma-10 from bottom to top. Layer Ma-3, contains a special tuff of russet colour which is designated as Azuki-tuff and is used as a key bed. The Pleistocene deposit above Azuki-tuf€ is conventionally designated as the upper division of the Osaka Group.

113

S. Murayama

Classifying the clay layers from their preconsolidation stresses, the clay in the alluvial layer belongs to the normally consolidated clay and that in the diluvia1 layers to the over- consolidated clay, Therefore, the alluvial layer has the most effect on the land subsidence

FIGUICE 1-8. Isopleth of land subsidence in Osaka area (1967-1968)

and the layer about 100 m deep, which is supposed to be a deposit of the hidden terrace, also has a serious effect on it. Furthermore, it seems that the upper division of the Osaka Group also has a similar effect on the subsidence.

114

Land subsidence in Osaka

(2) GENERAL PROPERTIES OP CLAY

The compressibility of Osaka clay was investigated on undisturbed samples obtained from deep borings; Boring No. 1 of 700 m depth in Northern Osaka, No. 4 of 500 rn depth, and No. 5 of 700 m depth in Eastern Osaka.

A-A Osaka City Eastern Osaka -~

*a-

$- m Sand

FIGURE 2-1. Geological profile along A-A line in figure 1-3

As shown in figure 2-3, most of the clays in Osaka can be classified into inorganic clays of high plastisity when their liquid limits and plastic indices are plotted on the plasticity chart.

Their activity numbers (A), or the ratios of the plastic index to the percentage of clay fraction, are shown in figure 2-4. As shown in this figure, most of clays belong to the active clay, since the A is greater than 1.25. In general, the larger the activity number of clay is, the greater becomes its volume change due to the decrease in its water content from the liquid limit to the shrinkage limit. Numbers affixed to the plots in figures 2-3 and 2-4 show the series numbers of marine clays shown in figure 2-1.

FIGURE 2-2. Boring logs of Borings No. 2 and No. 5

S. Murayama

Liquid limit WI (n)

FIGURE 2-3. Liquid limit and plastic index of Osaka clays plotted in plastisity chart

The consolidated volume (A V/V) of the normally consolidated clay due to the stress (p, + dp) can be represented by the following equation

A V c, p + d p V l+e P C

J = - -log _1, where:

pc e C, the compression index.

The values of C, obtained by standard oedometer tests are plotted against liquid limit (w,) as shown in figure 2-5. From this figure the following rough relation may be obtained on an average,

the preconsolidation stress of the clay; the void ratio of the clay before consolidation, and

C, = 0.017 (w, - 37).

o Percent clay fraction c2 JI %

FIGURE 2-4. Actiuity numbers of Osaka clays

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Land subsidence in Osaka

0" X O

o

.- c

.- s o

Ol + Alluvial clay

1.8- o Diluvial clay) No ' 4 Alluvial clay ii Diluvial clay) No + Alluvial clay 3 Diluvial clay) No

1.6- * ,

.i 1.4-

1.2-

ID-

%- o

Q2

Liquid limlt WI

FIGURE 2-5. Relationship between compression index C, and liquid limit ule

M- p, y - I

Figure 2-6 shows the distribution of preconsolidation stress, p,, plotted against the depth from the ground surface. In this figure, p, seems to increase in proportion to the depth and is generally larger than the present effective over-burden pressure. However, the plots of p, for the alluvial clays existing shallower than about 20 m lie on the over-

100-

- 200- f a u

300-

400-

Prscosolidation pressure pc '%'&, a IO 20 30 40 50

I l I I I

.' NO I \;c o NO 4

\ \ * \ O *

' 0 9 ', . \ B

I

FIGURE 2-6. Distribution of preconsolidation stress measured by oedometer tests

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S. Murayama

TABLE 2.1. Generalized stratigraphy of the Osaka district

Age Formation

Recent Alluvium ~

Late Terraces Pleistocene Hidden Terraces ?

Middle-Early Osaka Group (Upper division) Pleistocene “ Azuki tuff”

Plio-Pleistocene Osaka Group (Lower division)

Pliocene Nijo Group Late Miocene Kobe Group

Basement complex (chiefly granitic)

burden pressure line. Therefore, it can be said that those clays shallower than about 20 m belong to the normally consolidated clay and those deeper than about 20,mJo the over-consolidated clay.

(3) THE EFFECT OF LAND SUBSIDENCE ON THE PRECONSOLIDATION STRESS

Generally, the total stress applied to a clay due to the over-burden load is supportediby both pore water pressure and effective stress, where the effective stress means the inter- particle stress of the clay. Therefore, if the pore water pressure is decreased due to the lowering of the artesian head, the effective stress of the clay is increased by the same amount as the decrease in the pore water pressure. Consequently, the clay is consolidated in proportion to the increase in the effective stress.

The maximum effective stress which was ever applied to a clay is termed the pre-con- solidation stress of the clay. This pre-consolidation stress of a clay located at a certain depth in the ground can be measured by the oedometer test on the undisturbed clay sample. From this stress, the pore water pressure, u, at that depth can be calculated as the difference between the total stress and this stress. Figure 2-7 shows the p,-line, or the

FIGURE 2-7. Distribution of pre-consolidation stress in a diluvia1 clay stratum of land-subsidedarea

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Land subsidence in Osaka

distribution curve of the pre-consolidation stress, of a diluvial clay at a location in Osaka City.

In figure 2-7, the “total stress line” is calculated from the wet densities of undisturbed soil samples, the “static water pressure line” is drawn from the origin whose level is taken at the original underground water surface before land subsidence, the “p,-line” is obtained by plotting the pre-consolidation stress pe leftwards from the “total stress line”, and the “AB-line” is drawn by connecting the underground water levels in the upper aquifer and the lower one at the time when samples were obtained. From this figure, it is suggested that the consolidation in the clay stratum, or the land subsidence, was in progress at that time as far as the artesian heads remained as they were. From the “p,-line”, moreover, the degree of consolidation at that time and the subsequent amount of settlement due to the future consolidation of the clay stratum can be calculated by applying the theory o consolidation under an assumed relative position of “ AB-line ”.

3. REVIEW OF RESEARCHES ON THE LAND SUBSIDENCE IN OSAKA

There has been a variety of research and investigation on land subsidence in Osaka. Some scientific results of these studies will be introduced in this section and publications are listed under “references” at the end of this paper.

The firsi investigation of the land subsidence in Osaka was performed by K. Wadachi as chief of the science section in the Natural Disaster Research Institute, Japan Science Promotion Society. H e established the instrumentation (fig. 4-4) to observe the land subsi- dence and the artesian head in the aquifer at Kujoh Park in 1938, and verified that there was a linear relationship between the rate of land subsidence and the head difference between the artesian head and a certain level. In order to analyse such a relationship, he and T. Hirono extended the basic equation of consolidation proposed by Terzaghi by assuming the clay skeleton as a Voigt body (1942). Y. Ishii, as a research engineer in the Ministry of Transportation, explored the Osaka

soil, mainly throughout the alluvial layer. Besides, he also analysed the basic equation of consolidation by assuming another mechanical model (1949). This model consists of the series connection of a elastic spring and a Voigt element with a Newtonian viscosity. By comparing the coefficients of the model with the results of soil testing on undisturbed samples obtained by the exploration boring, he described the characteristics of land subsidence in Osaka.

As for the rheological property of clays, S. Murayama and T. Shibata proposed a new model consisting of an independent Hookean spring connected in series with a modified Voigt element, with the viscosity element in the Voigt model being deduced statistically by applying the rate process. The increase in the elasticities of the proposed model due to the consolidation was also analysed (1958, 1964).

Besides the above stated research on the role clay layers have in land subsidence, the influence of sandy aquifers on the Iand subsidence was investigated by S. Hayami, a pro- fessor in Kyoto University. He observed the variation in the modulus of compressibility of the sandy aquifer through repetitious pumping tests on the well in the field (1952-1955). Because the modulus of compressibility of the sandy aquifer during the draw-down period of the underground water level was found to be larger than that of the recovery period, he supposed that the sandy aquifer was compacted by the fluctuation in the artesian head. In order to verify these conclusion he and K. Akai performed model tests using a tank of large scale (1956, 1957).

From the investigations and researches stated above, it can be confirmed that the land subsidence in Osaka is caused by the excess withdrawal of underground water. However, the mechanism of the land subsidence is not so simple because two main causes of land subsidence seem to exist in Osaka; the consolidation of clayey layer due to lowering of

11 9

S. Murayama

artesian head and the repetitious compaction of the loose sandy aquifer due to the fluctu- ation of the artesian head.

Other research on land subsidence in Osaka is the geological investigation of the subsurface structure of Osaka Basin. This is being performed by S. Matsushita, a pro- fessor in Kyoto University, J. Iwatsu, N. Ikebe and J. Takenaka, professors in Osaka City University.

4. FIELD OBSERVATIONS AND RESULTS OBTAINED In order to investigate the real state of the land subsidence, the following observatims and surveys have been performed.

(1) LEVELING OF LAND SUBSIDENCE AREA

First order precise leveling of Osaka City was begun in 1885 by the Japanese Government, as a part of a survey net covering whole area of Japan. For this leveling, 12 bench marks were set in Osaka City at first. After land subsidence was noticed, more bench marks were established and additional leveling has been performed by the Municipal Authorities since 1934. After land subsidence extended to the surrounding regions around the City, the leveling of these regions has been performed by the Osaka Prefectural Government. At present, 576 bench marks in all are set in the whole area as listed in table-4.1. Such leveling is generally performed every winter, because the rate of land subsidence shows the least amount in winter and such tendency may help to promote the accuracy of the leveling. As the result of this leveling, various kinds of contour maps, annual .variation curves of land subsidence, etc. have been produced.

TABLE 4-1. Bench marks in Osaka

Name of region Nos. of B.M.

Osaka city 239

Northern Osaka 84

Eastern Osaka 136

Southern Osaka 117

Total 516

(2) SURVEY ABOUT PUMPING WELLS

A survey of pumping wells (their location, their purpose, numbers, quantity of water pumped, their depth, etc.) was performed. One of the results in Osaka City is shown in figure 4-1. The decrease in use of underground water since 1962 is shown in figure 4-1 and is mainly due to the effects of regulation and the law. The results of a survey in 1966 for the whole area of Osaka, except Osaka City, is shown in figure 4-2. Of the total quantity of water used (200 million cubic meters), the water used for buildings (mainly for air conditioning) represents 47 percent. Figure 4-3 shows the monthly variation of water quantity pumped from wells in Eastern Osaka in 1968, showing a peak in August.

(3) GEOLOGICAL AND SOIL-MECHANICAL INVESTIGATION IN OSAKA

Explanation about this subject is described in section 2.

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Land subsidence in Osaka

yeor

~~ ~

FIGURE 4-1. Nuinbers of wells and amount of ground water pumped

I

-2ûOm-250m (54) 3%

FIGURE 4-2. Distribution of depth of wells and quantity of under ground water pumped (Whole area of Osaka except Osaka City)

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S. Murayama

- E 2 0

E

f

E 9

0 IO X - f

o I 2 3 4 5 6 7 8 9 IO I I 12 ronth

FIGURE 4-3. Monthly variation of water quantitypumped from wells in 1968 (Whole area of Osaka except Osaka City)

(4) OBSERVATION OF SUBSIDENCE OF SOIL STRATA AND GROUND WATER LEVEL

The subsidence of soil strata and the ground water level of an aquifer are observed by the instrumentation whose schematic sketch is shown in figure 4-4. This instrumentation consists of two concentric steel pipes, of about 30 c m and 10 c m in diameters, inserted into a vertical bore hole. These pipes are fixed in a sandy aquifer. The bottom part of the outer pipe is perforated to make a strainer. If the land subsidence occurs in the soil strata between the ground surface and the aquifer, these steel pipes stick out of the ground.

Inner steel pip

I Nd iteel pipa audace

FIGURE 4-4. Outline of observation apparatus

Therefore, the relative displacement between the inner steel pipe and the ground surface, measured by a gage set near the head of inner pipe, represents the amount of land subsi- dence from the surface to the bottom of the steel pipe. An automatic recording gage of a certain magnification is usually used for the displacement gage. The underground water level can be measured by the automatic water gage placed between the inner pipe and outer one Figure 4-5 shows an example of continuous records measured automatically at the Kujoh observatory in August 1957. The locations of such observatories have been shown in figure 1-5, and their instrumentations in table 4-2.

If the instrument pipes are set on the aquifers of various depths, the amounts of land subsidence between the pipes of various depths can be observed. Examples of such observed results are shown in figure 4-6.

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Land subsidence in Osaka

L a n d subsidence

Groud water level

FIGURE 4-5. Records of land subsidence and ground water level measured at Kujoh Park I957 (25thlAug. was Suuday)

TABLE 4-2. Number of observatories in Osaka

Kind of Observation Nos.

Observatories for Land Subsidence 29

Observatories for Ground Water Level 43

1947 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 -2 -.

FIGURE 4-6.1. Vertical distribution of land subsidence in Western Osaka (at Kujoh Park, 1947-1968)

123

S. Murayama

at Nongo Yair

Subldra b h T k %rI<i~

__ Wildrr. blow Tk Surlos. - Suhlbnr bl0. n. s u i l a i

FIGURE 4-6.2. Vertical distribution of land subsidence in Eastern Osaka (1965-1968)

124

Land subsidence in Osaka

(5) INVESTIGATION OF AQUIFERS AND GROUND WATER

By using test wells in the field, the permeability and compressibility of aquifers were investigated under the guidance of Prof. S. Hayami. Investigation of a recharge well was also tested in the field.

FIGURE 5-2. Supply area of industrial water works

The ability of a well to supply ground water was sometimes determined by a pumping test of underground water. In this test, the ability of the well to supply water was determined by the quantity of water pumped, which corresponded to the inflective point of the curve representing the relation between quantity of water and water level during the test.

Since the underground water of different sources contains different amounts of soluble chemicals, the underground water system in Osaka was investigated by the examination of chemical analysis of the ground water.

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S. Murayama

5. R E G U L A T I O N A G A I N S T THE USE OF GROUND WATER AND ITS EFFECTS ON THE LAND SU B S I D E N C E %.

In order to prevent land subsidence the use of underground water had to be regulated. Besides the regulation, the river water had to be substituted for the underground water by establishing the industrial water works.

FIGURE 5-3.1. Isopleth of amount of land subsidence in Osaka (1960)

The regulation in Osaka City was decided first by the Industrial Water Law enacted in 1956 in order to prohibit constructing new wells. Afterwards, this law and its regulated district were reformed to be severer. According to the existing law, reformed in 1962, the districts regulated against the use of ground water for industrial purposes are shown in

126

Land subsidence in Osaka

figure 5-1. In each district, the maximum sectional area of a newly establishing pumping- tube and the minimum depth of a well were limited as described in figure 5-1. Figure 5-2 shows the present water supply area of industrial water in Osaka. The quantity supplied has previously been shown in figure 1-2.1.

FIGURE 5-3.2. Isopleth of amount of land subsidence in Osaka (1962)

As the underground water began to be used not only for industriai purpose but also for the buildings, the regulation of ground water for buildings was enacted in 1959 for a certain part of Osaka City. But the regulated district was extended to the entire City in 1962. The cooling tower method was substituted for use of underground water for buildings.

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S. Murayama

The effect of these regulations on the land subsidence in the City can be observed in figure 5.3.1 to 5.3.4. The regulations in the Eastern and Northern regions were described previously in section 1.

(unit :em

FIGURE 5-3.3. Isopleth of amounr of land subsidence bn Osaka (1964)

6. ACKNOWLEDGEMENT AND INFORMATION This paper introduces data on the land subsidence in Osaka obtained through various ñelds and many investigators. Data in Sections 1,4 and 5 were obtained from the engi- neers in the Osaka Prefectural Government and the Osaka Municipal Office, The contents of Section 2. (1) and 2. (2) have been summarized by Dr. J. Takenaka, Prof. in Osaka City University and Mr. N. Yagi, Assist. Prof. in Kyoto University respectively. The writer wishes to express his deep appreciation to them for their sincere co-operation.

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Land Jubsidence in Osaka

As this paper is too short to report sufficient data on the land subsidence in Osaka, another more detailed report is now being prepared for distribution at the Symposium by the editorial committee, which consists of the engineers in the Osaka Prefectural Government and the Osaka Municipal Office and the researchers participating in the investigations.

Scalo : In 0 1 2 3 4

FIGURE 5-3.4. Isopleth of amount of land subsidence in Osaka (1966)

REFERENCES

WADACHI, K. and HIRONO, T. (1942): Land Subsidence in Western Osoaka, Part 3 (Japanese), Bulletin of Natural Disaster Research Inst., No. 5.

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Tatsuso Okumura

ISHII, Y. (1949): Investigation on land subsidence in Osaka (Japanese), Report of the Technical Committee of Port Osaka.

MURAYAMA, S. and SHIBATA, T. (1958): O n the rheological characters of clay, Part 1, Bulletin of Disaster Prevention Research Inst. Kyoto Uniu., No. 26.

MURAYAMA, S. and SHIBATA, T. (1964): Flow and stress relaxation of clays, Proceedings of IUTAM Symposium on Rheology and Soil Mechanics, Grenoble, 1964, Springer Verlag, pp. 99-129.

HAYAMI, S. (1952-1955): Variation of artesian head and land subsidence in Osaka and Amagasaki City, Part I-VI11 (Japanese), Reports of the Technical Committee of Port Osaka.

HAYAMI, S. and AKAI, K. (1956, 1957): A hydraulic experimental research for the variation of ground-water pressure in artesian aquifers and the subsidence of ground, Part 1 and 2 (Japan- ese), Anuals of Disaster Preuention Research Inst. Kyoto Uniu.

ANALYSIS OF LAND SUBSIDENCE IN NIIGATA

Tatsuro QKUMURA

Port and Harbour Research Institute, Yokosuka, Japan

ABSTRACT The land subsidence in the Niigata area was analyzed by means of consolidation theory.

Pumping the underground water from the deep sandy layers for extracting mcthane gas caused a time-deprndent loading to the clay layers. An analytical solution of consol- idation under the load increasing linearly with time was obtained in terms of the vcrticai strain. A numerical method of analysis for the load decreasing linearly and then becoming constant was developed considering the difference in values of soil constants for consoli- dation and for rebound respectively. These two methods were combined for analyzing the subsidence in the Niigata area, and the results compared favourably with the observed sc ttlement.

RESUME L’affaissement du sol à Niigata a été analysé par le moyen de la théorie de consoli-

dation. Le pompage de l’eau souterraine à partir de la couche profonde sableuse pour extraire le gas méthane a causé la mise en charge, variable avec le temps, de ia couche de l’argile. Une solution analytique de la consolidation sous la charge augmentant avec le temps est obtenue en ce qui conceme !a déformation verticale. Une méthode numérique d’analyse de la charge augmentant d’abord linéairement et devenant ensuite constante est développée en tenant compte de la différence dans les valeurs des constantes du sol à la consolidation et au regonflement. Ces deux méthodes sont combinées pour analyser le tassement à Niigata, et les résultats ont été favorablemcnt comparés avec les valeurs observées.

1. INTRODUCTION

Niigata City and the neighbouring area have suffered a severe land subsidence since the 1950’s. The greatest rate of settlement of 53 cm per year was observed around the Port of Niigata in the period of 1958-1959 as shown in figure 1 (1st District Bureau for Pori Construction et al., 1963). Many of the port and coast facilities, river and road embank- ments, and farms and factories had gone out of use.

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